1. Field of the Invention
The present invention is directed generally to spring brake actuators, and more specifically to a spring brake actuator with a caging bolt bearing.
2. Description of Related Art
A pneumatic brake system for a large, heavy-duty vehicle such as a bus, truck, semi-tractor, or trailer typically includes a brake shoe and drum assembly which is actuated by an actuator that is operated by the selective application of compressed air. Conventional pneumatic spring brake actuators have both a service brake actuator for actuating the brakes under normal driving conditions by the application of compressed air and a spring-type emergency brake actuator which actuates the brakes when air pressure has been released from a pressure chamber. The emergency brake actuator, or spring brake, includes a strong compression spring which applies the brake when air is released.
There are two main types of pneumatic brake actuators, piston type actuators and diaphragm type actuators. In the diaphragm type brake actuator, two pneumatic diaphragm brake actuators are typically arranged in a tandem configuration, which includes a pneumatic service brake actuator for applying the normal operating brakes of the vehicle, and a spring brake actuator for applying the parking or emergency brakes of the vehicle. Both the service brake actuator and the spring brake actuator include a housing having an elastomeric diaphragm dividing the interior of the housing into two distinct fluid chambers. The piston type brake actuator is substantially similar to the diaphragm type, except that instead of a diaphragm, a piston reciprocates in a cylinder for applying the normal and/or parking brakes of the vehicle.
In a typical service brake actuator, the service brake housing is divided into a pressure chamber and a pushrod chamber. The pressure chamber is fluidly connected to a source of pressurized air and the pushrod chamber mounts a pushrod that is coupled to the brake assembly. The introduction and exhaustion of pressurized air in to and out of the pressurized chamber reciprocates the pushrod in to and out of the housing to apply and release the operating brakes.
In a typical spring brake actuator, the spring brake section is divided into a pressure chamber and a spring chamber by a diaphragm. A pressure plate is positioned in the spring chamber between the diaphragm and a strong compression spring, whose opposing end abuts the housing of the section. In one well-known configuration, an actuator tube extends through the pressure plate, through the diaphragm, into the pressure chamber, and through a dividing wall separating the spring brake actuator from the service brake actuator. The end of the actuator tube is fluidly connected to the pressure chamber of the service brake actuator.
When applying the parking brakes, the spring brake actuator pressure is discharged from the pressure chamber and the large force compression spring pushes the pressure plate and the diaphragm toward the dividing wall between the spring brake actuator and the service brake actuator. In this position, the actuator tube connected to the pressure plate is pushed for applying the parking or emergency brakes and thus immobilizing the vehicle. To release the parking brake, pressurized air is introduced into the pressure chamber of the spring brake actuator to expand the pressure chamber, move the diaphragm and pressure plate toward the opposing end of the spring brake actuator housing, and compress the compression spring.
A caging bolt or tool is used to cage the strong compression spring when, for example, the brake must be manually released. It is known to permanently install a caging bolt in a brake actuator. In one such actuator, a threaded bolt extends through a threaded opening in the actuator housing and into a hollow interior of the spring brake actuator tube, which is affixed to the pressure plate. As the emergency brake or parking brake is applied, the pressure plate and actuator tube reciprocate relative to the bolt. This reciprocation over time causes the threads of the bolt to wear. To prevent damage to the caging bolt threads, a bearing is typically positioned within the pressure plate around the caging bolt. The bearing allows the pressure plate and actuator tube to reciprocate relatively smoothly with respect to the caging bolt. Conventionally, the bearing is positioned within an opening in the pressure plate and retained there by the actuator tube which is press fit into the pressure plate opening. The bearing may also be bonded to the pressure plate.